Add matscipy as an ASE v4 neighbour-list plugin

[jameskermode]

Summary

Exposes matscipy's compiled neighbour list as a user-selectable neighbour-list backend for ASE 4 (ase._4), via the single ase.plugins entry-point group. When matscipy is installed, ASE callers can opt in explicitly:

from ase.neighborlist import get_neighbor_list, available_neighbor_list_backends
available_neighbor_list_backends()      # {'ase': True, 'matscipy': True, ...}
nl = get_neighbor_list("matscipy")      # the compiled backend
i, j, d, D, S = nl("ijdDS", atoms, 5.0)

Selection is explicit and never automatic — this only makes the backend available under the name "matscipy"; ASE still defaults to its built-in "ase" backend.

What's here

• matscipy/_ase_plugin.py — a thin adapter matching ASE's NeighborListFunction contract (fn(quantities, atoms, cutoff, *, self_interaction=False)). matscipy's neighbour_list already returns the same (i, j, d, D, S) flat arrays and quantity letters; the adapter only adds the self_interaction keyword, rejecting self_interaction=True (which matscipy does not support) rather than silently differing. The heavy matscipy.neighbours import is lazy (only on call), and the __ase_plugins__ registration is guarded so installing matscipy alongside an ASE that predates the v4 plugin API registers nothing instead of breaking plugin discovery.
• pyproject.toml — declares the ase.plugins entry point (matscipy_neighbours = "matscipy._ase_plugin").
• matscipy/meson.build — installs the new module.

Notes / dependencies

• Targets ASE's in-flight v4 plugin system (the ase.plugins entry-point group + NeighborListPlugin type). On current released ASE the guard makes this a no-op, so it is safe to merge ahead of that landing.
• Validated against an ASE branch carrying the v4 NeighborListPlugin: matscipy's edge sets are byte-identical to ASE's reference backend across fcc, hcp, and mixed-pbc slab systems (cutoffs 3 and 5 Å), the D = pos[j] - pos[i] + S @ cell identity holds, and matscipy works through ASE's backend-agnostic Verlet skin-reuse wrapper.

🤖 Generated with Claude Code

[jameskermode]

Closing without merging. We're integrating the neighbour-list backend via the standalone matscipy-neighbours package instead of adding the ASE plugin to full matscipy — see libAtoms/matscipy-neighbours#2, which adds pip packaging (scikit-build-core) plus the same ase.plugins NeighborListPlugin adapter there.

Rationale: matscipy-neighbours is a slim, separately-installable extraction of matscipy.neighbours (same (i, j, d, D, S) API, optional CUDA/HIP GPU backend), so ASE users can opt into the compiled neighbour list without pulling in all of matscipy. In a fresh benchmark it was also the fastest CPU backend measured (~3× full matscipy, lowest memory). Thanks!